She’s clearly figured out the singles-bar body switch. The contrast between Marc & Bax’s behavior now, versus “Marc” & “Bax” at the bar. The broad hints that were dropped in conversation with “Marc”, e.g. “…like in another body…” “Marc” not having Marc’s distinctive speech pattern. And bringing up Ice Princess and ballet dancer was no accident, it was a test (I can’t find what it was referring back to, though) and Bax’s response was a dead giveaway…

Di *knows*, as all top AEGIS agents do, about that body-swapping machine…